Organic luminescent materials, coating solution using same for organic emitting layer, organic light emitting device using coating solution and light source device using organic light emitting device

ABSTRACT

It is an object of the present invention to provide an organic light-emitting device which can emit white light by easily controlling dopant concentrations. The organic light-emitting device has a first electrode ( 112 ) and second electrode ( 111 ) which hold a light-emitting layer ( 113 ) in-between, wherein the light-emitting layer contains a host material ( 104 ), red-light-emitting dopant ( 105 ), green-light-emitting dopant ( 106 ) and blue-light-emitting dopant ( 107 ), the red-light-emitting dopant containing a first functional group for transferring the dopant toward the first electrode and the green-light-emitting dopant containing a second functional group for transferring the dopant toward the second electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 12/858,470, filed Aug. 18, 2010, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an organic luminescent material,coating solution using the organic luminescent material for organiclight-emitting layers, organic light-emitting device using the coatingsolution and light source device using the organic light-emittingdevice.

BACKGROUND OF THE INVENTION

The methods for producing organic light-emitting devices (LEDs) broadlyfall into two categories, vacuum vapor deposition and coating. Thecoating method has advantages of easily producing large-area films andhigh material utilization factor. In order to apply the coating method,development of devices with a single light-emitting layer is demanded,because it is necessary to reduce number of organic LED layers.

For organic white-light-emitting devices with a single light-emittinglayer, Patent Document 1 discloses an organic EL device with a singlelight-emitting layer of a composition of at least (a) polymer and (b)compound for forming a light-emitting center, placed between electrodes.The composition contains an electron transfer material and hole transfermaterial in a well-balanced manner. The polymer itself emits blue orshorter wavelength colors, and is dispersed with at least two species ofcompounds for forming a light-emitting center, each individuallyemitting a color, where a combination of these compounds is selected insuch a way that the organic EL device as a whole emits white color.

Recently, organic light-emitting devices have been attracting attentionas planar light sources of the next generation. An organiclight-emitting device exhibits excellent characteristics ofspontaneously emitted light, wide view angle, high responsiveness andhigh color rendition. One example of organic light-emitting devicecomprises a glass substrate which supports a transparent electrode(e.g., of ITO), organic layer composed of hole transport layer,light-emitting layer, electron transport layer and so on, and repellerof low work function, where light is emitted from the back side of thesubstrate after passing through the electrode.

The methods for producing organic light-emitting devices include vacuumvapor deposition, ink jetting and printing. Vacuum vapor depositionheats and evaporates an organic material under a vacuum and deposits thevapor on a substrate to form the film thereon. It can simply produce anorganic light-emitting device of desired structure because of itscapability of controlling film thickness and concentration bymanipulating deposition rate. However, it involves disadvantages of lowmaterial utilization factor and difficulty in increasing substrate size.Ink jetting or printing is expected to be a low-cost wet method, becauseof its advantages of high material utilization factor and easily forminglarge-area device, although involving a disadvantage of difficulty informing laminated structures because solubility of an underlyingmaterial must be controlled for forming a laminated structure.

An organic light-emitting device must emit white color when used as alight source. It is necessary for the device to emit three colors ofred, blue and green in order to realize white color of high rendition.

The wet methods proposed so far for producing organicwhite-light-emitting devices include mixing three species of dopantseach emitting red, blue or green color in a light-emitting layer. Forexample, Patent Document 1 discloses a method which uses polyvinylcarbazole as a host material for light-emitting layer which is dispersedwith red-, blue- and green-light-emitting dopants at a low concentration(0.01 to 5% by mol) to realize white color.

-   [Patent Document 1] JP-A-9-63770

BRIEF SUMMARY OF THE INVENTION

The conventional method for producing an organic light-emitting deviceinvolves a disadvantage of difficulty in emitting white color by easilycontrolling red-, green- and blue-light-emitting dopant concentrationswithout causing their phase separation.

The objects of the present invention are to provide an organicluminescent material capable of being controlled for dopantconcentrations, coating solution using the organic luminescent materialfor organic light-emitting layers, organic light-emitting device usingthe coating solution and light source device using the organiclight-emitting device.

The conventional coating method for producing an organic light-emittingdevice involves a disadvantage of difficulty in controlling dopantconcentrations because of their very low concentrations, about 0.02% bymol with a green-light-emitting dopant, 0.02% and 0.015% by mol with ared-light-emitting dopant.

The other objects of the present invention are to provide an organicluminescent material easily emitting white light, coating solution usingthe organic luminescent material for organic light-emitting layers,organic light-emitting device using the coating solution and lightsource device using the organic light-emitting device.

One of the features of the present invention for solving the aboveproblems is an organic light-emitting device with first and secondelectrodes which hold a light-emitting layer in-between, wherein thelight-emitting layer contains a host material, red-light-emitting dopantand blue-light-emitting dopant, the red-light-emitting dopant containinga first functional group for transferring the dopant toward the firstelectrode.

Another feature of the present invention is a coating solution forproducing the light-emitting device for the organic light-emittingdevice, the solution containing a solvent, host material,red-light-emitting dopant and blue-light-emitting dopant.

Still another feature of the present invention is an organic luminescentmaterial for producing the light-emitting device for the organiclight-emitting device, the material containing a host material,red-light-emitting dopant, blue-light-emitting dopant andgreen-light-emitting dopant.

Still another feature of the present invention is an organiclight-emitting device with first and second electrodes which hold alight-emitting layer in-between, wherein the light-emitting layercontains a host material, red-light-emitting dopant andgreen-light-emitting dopant, the red-light-emitting dopant being presentin the light-emitting layer at graded concentrations.

Still another feature of the present invention is a coating solution forproducing the light-emitting device for the organic light-emittingdevice, the solution containing a solvent, host material,red-light-emitting dopant and green-light-emitting dopant.

Still another feature of the present invention is an organic luminescentmaterial for producing the light-emitting device for the organiclight-emitting device, the material containing a host material,red-light-emitting dopant, blue-light-emitting dopant andgreen-light-emitting dopant.

Still another feature of the present invention is a light source deviceusing the organic light-emitting device including the organiclight-emitting device.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

ADVANTAGES OF THE INVENTION

The present invention provides an organic luminescent material capableof emitting white light, produced by easily controlling dopantconcentrations, coating solution using the organic luminescent materialfor organic light-emitting layers, organic light-emitting device usingthe coating solution and light source device using the organiclight-emitting device. The other challenges, structures and advantageswill be clarified by the description of the preferred embodimentsdescribed below.

The present invention also provides an organic luminescent materialcapable of easily emitting white light, coating solution using theorganic luminescent material for organic light-emitting layers, organiclight-emitting device using the coating solution and light source deviceusing the organic light-emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy diagram of each component in thewhite-light-emitting device.

FIG. 2 is a cross-sectional view of one embodiment of the organiclight-emitting device of the present invention.

FIG. 3 is a cross-sectional view of one embodiment of thewhite-light-emitting device of the present invention.

FIG. 4 is an energy diagram of the white-light-emitting device preparedin Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in detail by referring to theattached drawings and the like.

The conventional coating method for producing an organic light-emittingdevice involves a disadvantage of difficulty in controlling dopantconcentrations because of their very low concentrations, about 0.02% bymol with a green-light-emitting dopant, 0.02% and 0.015% by mol with ared-light-emitting dopant. Moreover, the device cannot exhibit asufficient light-emitting efficiency, because of insufficient energytransfer between the dopants and insufficient containment of the carrierin the light-emitting region.

Moreover, the conventional coating method for producing an organiclight-emitting device involves disadvantages of insufficientlight-emitting efficiency and difficulty in controlling dopantconcentrations, resulting from transfer of energy between the dopants.Excitation energy of light-emitting dopant decreases in the descendingorder of blue-light-emitting, green-light-emitting andred-light-emitting dopants, by which is meat that energy transfer tendsto occur from a blue-light-emitting dopant to a green-light-emittingdopant and from a green-light-emitting dopant to a red-light-emittingdopant. In a light-emitting structure with the three species of dopants,they are present close to each other, tending to cause energy transferbetween the different species of dopants. A blue-light-emitting dopantwill be transferred eventually to a red-light-emitting dopant of thelowest excitation energy when energy transfer can occur, depleting theblue-light-emitting dopant with the result that the device cannotefficiently emit while light. For the device to sufficiently emit bluelight, it is necessary to greatly decrease green-light-emitting andred-light-emitting dopant concentrations.

The organic light-emitting layer as one embodiment of the presentinvention contains a host material and a dopant having a substituentwhich works to localize the dopant during the film-making step. Thedopant is localized in the vicinity of the light-emitting layer surfaceor interface with the underlying layer. Being localized in the vicinityof the electrode means that the dopant is present in the layer at ahigher concentration in the vicinity of the electrode. As a result, thelight-emitting layer prepared by the wet method has a functionsubstantially equivalent to that of a three-layered light-emittinglayer. In such a structure, distance between the dissimilar dopantsincreases except in the vicinity of the interface, decelerating energytransfer between the dissimilar dopants, which facilitates control ofdopant concentrations and formation of a white-light-emitting device.

FIG. 2 is a cross-sectional view of one embodiment of the organicwhite-light-emitting device of the present invention. The device has asubstrate 110 which supports a lower electrode 111 as the secondelectrode, organic layer 113 and upper electrode 112 as the firstelectrode in this order from the substrate. The device is of bottomemission type in which light emitted by a light-emitting layer 103 istaken out from the lower electrode 111 side. The lower electrode 111 isthe transparent electrode working as the anode and upper electrode 112is the repeller working as the cathode. The organic layer 113 may be ofa single-layer structure with the light-emitting layer 103 alone ormulti-layer structure with one or more other layers selected from thegroup consisting of electron injection layer 109, electron transportlayer 108, hole transport layer 102 and hole injection layer 101. Theorganic light-emitting layer illustrated in FIG. 1 makes a light sourcedevice, when provided with a driving circuit, case and so on.

The light-emitting layer contains a host molecule and dopant moleculewhich contains a red-light-emitting, green-light-emitting andblue-light-emitting dopants. A material for forming the light-emittinglayer 103 contains a host molecule, and red-light-emitting,green-light-emitting and blue-light-emitting dopants. However, thematerial may not necessarily need a green-light-emitting dopant, if itcan emit white light. Each of the dopants is localized in thelight-emitting layer 103 to form a pseudo laminated structure. First,the light-emitting layer structure is described.

<Phase Separation>

When red-light-emitting, green-light-emitting and blue-light-emittingdopants are mixed with each other to form a single white-light-emittinglayer, one species of the dopant is surrounded by one or two species ofthe dissimilar dopants, with the result that excitation energy istransferred from the dopant to the adjacent dissimilar dopant at acertain probability. For example, when the blue-light-emitting dopant ispresent adjacently to the green-light-emitting or red-light-emittingdopant, excitation energy is transferred from the blue-light-emittingdopant to the dopant of lower energy, green-light-emitting orred-light-emitting dopant, making it difficult for the single layer toemit white light. Even the coating method should produce a deviceefficiently emitting white light even when each of the dopants ispresent at a high concentration, if it could realize spontaneousphase-separation between the dissimilar dopants to separate dopants oflower energy from each other. The preset invention realizes thespontaneous phase-separation by incorporating each of the dopants withan adequate functional group.

<Host Material>

Examples of the preferable host material 104 include carbazole, fluoreneand arylsilane derivatives. The host material preferably has anexcitation energy sufficiently higher than that of theblue-light-emitting dopant to efficiently emit light, where excitationenergy is determined by photoluminescence spectra.

<Red-Light-Emitting Dopant>

Examples of preferable materials for the red-light-emitting dopant 105include those having a major skeleton of rubrene,(E)-2-(2-(4-dimethylamino)styryl)-6-methyl-4H-pyran-4-ylidene)malononitrile(DCM), its derivative, iridium complex (e.g.,bis(1-phenylisoquinoline)(acetylacetonate) iridium (III)), osmiumcomplex or europium complex, of which the iridium complex represented byFormula 1 is more preferable viewed from light-emitting characteristics,more preferably it has an acetylacetonate site, wherein X1 is anN-containing aromatic hetero ring, and X2 is an aromatic hydrocarbonring or aromatic hetero ring.

Examples of the aromatic hetero ring represented by X1 includequinoline, isoquinoline, pyridine, quinoxaline, thiazole, benzothiazole,oxazole, benzoxazole, indole and isoindole rings. Examples of thearomatic hydrocarbon ring or aromatic hetero ring represented by X2include benzene, naphthalene, anthracene, thiophene, benzothiophene,furan, benzofuran and fluorene rings. When the upper electrode serves asthe cathode and lower electrode as the anode, the red-light-emittingdopant, having a first functional group for transferring the dopanttoward the first electrode, is preferably located in the upper portionof the light-emitting layer (surface side), which localizes the dopanton the upper electrode side. Examples of the first functional group Y1or Y2 to be added to the acetylacetonate site to transfer the dopanttoward the film surface side during the film-making step includefluoroalkyl, perfluoroalkyl, alkyl (of 10 carbon atoms or more),perfluoropolyether and siloxy (Si—O—Si—) groups. The red-light-emittingdopant 105 may contain one or more of these functional groups. The groupmay be introduced to the major skeleton directly, as illustrated byFormulae 2 and 3, or indirectly via an amido or ester bond, asillustrated by Formula 4.

<Green-Light-Emitting Dopant>

Examples of preferable materials for the green-light-emitting dopant 106include those having a major skeleton of coumarin, its derivative,iridium complex (e.g., tris(2-phenylpyridine) iridium, Ir (ppy)3). Whenthe upper electrode serves as the cathode and lower electrode as theanode, the green-light-emitting dopant is preferably located in thelower portion of the light-emitting layer. The dopant contains a secondfunctional group for transferring the dopant toward the lower electrodeor hole transport layer. The second functional group varies depending onthe underlying layer to which the dopant is transferred during thefilm-making step. When the underlying layer is the hole transport layer,the group should have a structure similar to the hole transport layer,e.g., phenylamino, oxazole or carbazole group, or hydrazone site. Thegreen-light-emitting dopant 106 may contain one or more of thesefunctional groups. When the underlying layer is the electrode of ITO ora metal, examples of the functional group include hydroxyl (—OH), thiol(—SH), carboxyl (—COOH), sulfo (—SO₃H) or bipyridyl group, or I, Br, Cl,F, SCN, CN, NH₂ or NO₂. The green-light-emitting dopant 106 may containone or more of these functional groups. The group may be introduced tothe major skeleton directly, as illustrated by Formula 5, or indirectlyvia an alkyl chain in consideration of the molecular size.

<Blue-Light-Emitting Dopant>

Examples of the major skeleton of the blue-light-emitting dopant 107include perylene, iridium complex (e.g.,bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium (III),FIrpic). The blue-light-emitting dopant may not necessarily need afunctional group. However, a structure incompatible with the underlyinglayer may be introduced to efficiently cause the phase separation.

<Containment of Carrier>

Let's consider that the phase-separation spontaneously occurs in each ofthe red-light-emitting dopant 105, green-light-emitting dopant 106 andblue-light-emitting dopant 107 to form the pseudo laminated structure,illustrated in FIG. 1. The green-light-emitting, blue-light-emitting andred-light-emitting dopants are arranged in this order from the anode inconsideration of the carrier conduction, determined by the highestoccupied molecular orbital (HOMO) energy and lowest unoccupied molecularorbital (LUMO) energy of each of the dopants. The HOMO energy isdetermined by photoemission spectroscopy, whereas the LUMO energy may bedetermined by finding the HOMO-LUMO differential energy from theabsorption spectra or directly by inverse photoemission spectroscopy.When the host material has a large HOMO-LUMO energy differential, andeach of the dopants has the HOMO and LUMO energies in the differentialrange and is present at an adequate concentration, the carrierconduction proceeds by hopping over the level of each of the dopants.When the absolute value of the LUMO energy of the blue-light-emittingdopant 107 is sufficiently higher than that of the green-light-emittingdopant 106, the electrons propagating over the LUMO of theblue-light-emitting dopant 107 hop to the LUMO of thegreen-light-emitting dopant 106 at a reduced probability, and mostlycontained in the blue-light-emitting dopant 107. When the differentialbetween the HOMO energy of the blue-light-emitting dopant 107 and thatof the green-light-emitting dopant 106 is relatively small, the holespropagating over the HOMO of the green-light-emitting dopant 106 can hopto the HOMO of the blue-light-emitting dopant 107, with the result thatrecombination of the carriers (electrons or holes) occurs on theblue-light-emitting dopant 107, to directly emit blue light, or theexcitation energy transfers toward the green-light-emitting dopante 106to emit green light. On the other hand, the holes can be containedbetween the red-light-emitting dopant 105 and electron injection layer109, to emit red light by the recombination of the injected electron.The red light emission can also occur when the excitation energytransfers from the blue-light-emitting dopant 106 orgreen-light-emitting dopant 107.

As discussed above, the present invention contains the carrier in thevicinity of each of the dopants, thus improving emission efficiency ofeach light of color and hence realizing a white-light-emitting device ofhigh efficiency.

Next, the other components are described. As discussed earlier, the holeinjection layer 101, hole transport layer 102, electron transport layer108 or electron injection layer 109 are not necessarily needed.

Examples of the preferable material for the hole injection layer 101include electroconductive polymers, e.g.,poly(3,4-ethylenedioxythiophene), PEDOT, and polystyrenesulfonate, PSS.Polypyrrole-base and tirphenylamine-base polymers are also useful. Theymay be used in combination of low-molecular-weight material.Phthalocyanine-base and starburst-amine-base compounds are alsoapplicable.

Examples of the material for the hole transport layer 102 include, butnot limited to, starburst-amine-base compound, stilbene derivative,hydrazone derivative and thiophene derivative. They may be used incombination.

The electron transport layer 108 is responsible for donating electronsto the light-emitting layer 103. Examples of the material for theelectron transport layer 108 includebis(2-methyl-8-quinolinolate)-4-(phenylphenolate) aluminium (Balq),tris(8-quinolinolate)aluminum (Alq3), oxadiazole derivative, triaszolederivative, fllerene derivative, phenanthroline derivative and quinolinederivative.

The electron injection layer 109 works to improve efficiency ofinjection of the electrons from the cathode to the electron transportlayer 108. Examples of the preferable material for the electroninjection layer 109 include, but not limited to, lithium fluoride,magnesium fluoride, calcium fluoride, strontium fluoride, bariumfluoride, magnesium oxide and aluminum oxide. They may be used incombination.

The material for the anode as the lower electrode 111 is not limited solong as it is transparent and having a high work function. The examplesinclude an electroconductive oxide, e.g., ITO or IZO, or metal of highwork function, e.g., thin Ag. The electrode can be patterned normally ona substrate, e.g., glass, by photolithography.

The cathode as the upper electrode 112 works to reflect light emittedfrom the light-emitting layer 103. Examples of the material for theupper electrode 112 include, but not limited to, LiF/Al laminate andMg/Ag alloy. LiF may be replaced by a Cs, Ba or Ca compound.

The coating solution of the present invention is composed of a hostmaterial, and red-light-emitting, green-light-emitting andblue-light-emitting dopants dissolved in an adequate solvent. Thecoating solution may not necessarily need a green-light-emitting dopant.The solvents useful for the present invention are not limited so long asthey can dissolve these components. The examples include aromatichydrocarbons (e.g., toluene), ethers (e.g., tetrahydrofuran), alcohols,fluorine-base ones. These solvents may be used in combination foradjusting solubility and drying speed of each component. For example, amixture of solvents of different boiling point may be used, wherein thehigher-boiling one is used as a poor solvent with the red-light-emittingdopant to accelerate transfer of the dopant to the film surface.Solubility of the solvent is determined by liquid chromatography.

The coating methods for forming the light-emitting layer include spincoating, casting, dip coating, spray coating, screen printing andink-jet printing, one of which is selected to form the light-emittinglayer.

The device structure is described taking a bottom emission typestructure as an example. However, the present invention is alsoapplicable to a top emission type structure with the upper electrodeworking as a transparent electrode so long as the upper and lowerelectrodes work as the respective cathode and anode.

The present invention is described in more detail by specific examples.It should be understood that these examples are intended to illustratesome aspects of the present invention and not to limit the invention.Those skilled in the art can make various variations and modificationswithout departing from the scope of the technical concept of theinvention described herein.

EXAMPLES Example 1 Synthesis of Exemplary Compound 1

First, the red-light-emitting dopant represented by the structuralformula (1) was synthesized as one of the major constituents of thepresent invention to produce the white-light-emitting device of thepresent invention.

The compound represented by Formula 6 was synthesized as the essentialintermediate for synthesizing the compounds represented by Formulae 2and 3 by the following procedure.

A 200 mL three-necked flask was charged with 0.718 g ofphenylisoquinoline dissolved in 30 mL of ethoxyethanol and 0.418 g ofiridium chloride dissolved in 10 mL of water, which were mixed with eachother in the flask. The mixture was heated at 120° C. for 10 hours underreflux in a nitrogen atmosphere, then cooled to room temperature, andtreated by evaporation. The resulting solid was washed with alcohol toproduce the compound represented by Formula 6.

(Synthesis of Compound Represented by Formula 2)

The compound represented by Formula 2 was synthesized by the followingprocedure.

A 200 mL three-necked flask was charged with 0.959 g of the intermediateA, 0.512 g of 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octsanedione,0.25 g of sodium carbonate and 30 mL of ethoxyethanol. The mixture washeated at 115° C. for 10 hours under reflux in a nitrogen atmosphere,then cooled to room temperature, and treated by evaporation. Theresulting solid was washed with water and hexane, and treated bysilica-gel column chromatography with a mixed solvent of ethyl acetateand hexane as the mobile phase to produce the compound represented byFormula 2. It had a molecular weight of 897, determined by massanalysis.

The compound represented by Formula 2 was dissolved in dichloromethaneand analyzed by fluorescent spectroscopy. It emitted red light having apeak wavelength of 617 nm.

A mixed film of the compound represented by Formula 2 and mCP as thehost material was formed on a quartz substrate by spin coating, with THFas a solvent used to keep the solid concentration at 1% by mass andexemplary compound concentration at 10% by mass based on mCP. Thecoating film was measured for contact angle with water, which can bedetermined by θ/2 method, tangential or curve-fitting method. It was92.1°. The films of individual mCP and the compound represented byFormula 2 as the reference samples had the contact angles of respective80.6 and 96.7°. It was considered that the compound represented byFormula 2 was not uniformly dispersed in mCP but distributed more in thesurface area because of the angle greatly changing with the mixingratio.

(Synthesis of Compound Represented by Formula 3)

The compound represented by Formula 3 was synthesized by the followingprocedure.

A 200 mL three-necked flask was charged with 0.959 g of the compoundrepresented by Formula 6, 0.706 g of1,1,1,2,2,3,3,7,7,8,8,9,9,9-tetradecafluoro-4,6-nonandione, 0.25 g ofsodium carbonate and 30 mL of butoxyethanol. The mixture was heated at150° C. for 20 hours under reflux in a nitrogen atmosphere, then cooledto room temperature, and treated by evaporation. The resulting solid wasdissolved in dichloromethane and filtered. The filtrate was separatedand washed with dichloromethane/water. The dichloromethane solution wasremoved and treated by evaporation. The resulting solid was washed withhexane and treated by alumina column gas chromatography withdichloromethane as the mobile phase to produce the compound representedby Formula 3.

<Preparation of Organic Light-Emitting Device>

Example 1 prepared the white-light-emitting device having a structureillustrated in FIG. 2, with the lower electrode of ITO and holeinjection layer of PEDOT, formed by spin coating, and hole transportlayer of a polymer. The organic light emitting layer was composed of mCP(1,3-bis(carbazol-9-yl)benzene) as the host material, iridium complex(bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III)) asthe blue-light-emitting dopant and the compound represented by Formula2, prepared above, as the red-light-emitting dopant in a weight ratio of100/5/1.

The host material, blue-light-emitting dopant and red-light-emittingdopant were dissolved in THF to keep the solid concentration at 1% bymass and red-light-emitting dopant at 0.46% by mol based on the solidcomponent, determined by liquid chromatography. The solution was used toform the organic light-emitting layer by spin coating. Then, theelectron transport layer of BAlq and Alq3 was formed by vacuum vapordeposition, and then the upper electrode of LiF/Al laminate was formed,to prepare the target organic light-emitting device.

A voltage was applied to the organic light-emitting device thus preparedto confirm that each of the red-light-emitting and blue-light-emittingdopants emitted light, as evidenced by the EL spectral pattern, and thatthe device emitted white light. The device was also prepared in the samemanner except that the red-light-emitting dopant contained nofluoroalkyl group for comparison. The device was confirmed to emit bluelight of decreased intensity and red light of increased intensity.

Example 2

Example 2 prepared the light-emitting layer in the same manner as inExample 1, except that mCP was used for the host material,bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium (III) wasused for the blue-light-emitting dopant, the compound represented byFormula 2 was used for the red-light-emitting dopant, and the compoundrepresented by Formula 5 or Ir(ppy)3 dissolved in THF was used for thegreen-light-emitting dopant.

A voltage was applied to the organic white-light-emitting device withthe green-light-emitting dopant of the compound represented by Formula 5to confirm that each of the red-light-emitting dopant,green-light-emitting and blue-light-emitting dopants emitted light. Forthe device with the green-light-emitting dopant of Ir(ppy)3, theblue-light-emitting and green-light-emitting dopants emitted light oflower intensity than the red-light-emitting dopant.

Example 3

FIG. 3 is a cross-sectional view of the organic white-light-emittingdevice prepared in Example 3. The device had an upper electrode 212 asthe first electrode, lower electrode 211 as the second electrode andorganic layer 213. The lower electrode 211 and upper electrode 212 maybe used as the respective first and second electrodes. The deviceillustrated in FIG. 3 comprised a substrate 210 which supported thelower electrode 211, organic layer 213 and upper electrode 212 in thisorder from the substrate. It was of bottom emission type in which lightemitted by a light-emitting layer 203 was taken out from the lowerelectrode 211 side. The lower electrode 211 was the transparentelectrode working as the anode and upper electrode 212 was the repellerworking as the cathode. The organic layer 213 was composed of a holeinjection layer 210, hole transport layer 202, light-emitting layer 203,electron transport layer 208 and electron injection layer 209. Theorganic layer 213 may not necessarily have the above structure. It maybe of a single-layer structure with the light-emitting layer 203 aloneor multi-layer structure having no hole transport layer 202. Moreover,the electron transport layer 208 may be a laminate of the electrontransport layer and blocking layer. The organic light-emitting layerillustrated in FIG. 3 makes a light source device, when provided with adriving circuit, case and so on.

The light-emitting layer 203 had a host material, and red-light-emittingdopant 205, green-light-emitting dopant 206 and blue-light-emittingdopant 207. The green-light-emitting dopant 206 may not be necessary solong as the layer 203 emit white light. Each of the red-light-emittingdopant 205 and blue-light-emitting dopant 207 were localized in thelight-emitting layer 203 to form a pseudo laminated structure. First,the light-emitting layer structure is described.

An iridium complex represented by Formula 1′ was used for thered-light-emitting dopant.

Another useful iridium complex is represented by Formula 2′.

In Formula 2′, X1 is an N-containing aromatic hetero ring, and X2 is anaromatic hydrocarbon ring or aromatic hetero ring. Examples of thearomatic hetero ring represented by X1 include quinoline, isoquinoline,pyridine, quinoxaline, thiazole, pyrimidine, benzothiazole, oxazole,benzoxazole, indole and isoindole rings. Examples of the aromatichydrocarbon ring or aromatic hetero ring represented by X2 includebenzene, naphthalene, anthracene, thiophene, benzothiophene, furan,benzofuran and fluorene rings. Examples of Y1 and Y2 includefluoroalkyl, perfluoroalkyl, alkyl (of 10 carbon atoms or more),perfluoropolyether and siloxy (Si—O—Si—) groups. The red-light-emittingdopant 205 may contain one or more of these functional groups. Thefunctional group of Y1 or Y2 works to lower surface energy and hence tolocalize the red-light-emitting dopant in the surface on the upperelectrode side in the light-emitting layer.

The functional group may be introduced to the major skeleton directly,as illustrated by Formula 2′, or indirectly via an amido or ester bond,to distribute red-light-emitting dopant in the light-emitting layer atgraded concentrations.

Other compounds having a similar functional group may be used. Theseinclude so-called phosphorescent dopants, e.g., osmium complex, europiumcomplex and platinum complex, and(E)-2-(2-4-(dimethylamino)styryl)-6-methyl-4H-pyran-4-ylidene)malononitrile(DCM).

Examples of the material for the host material 204 in the light-emittinglayer 203 include mCP (1,3-bis(carbazol-9-yl)benzene). The othermaterials include carbazole, fluorine and arylsilane derivatives. Thehost material 204 preferably has a sufficiently higher excitation energythan the blue-light-emitting dopant, in order to efficiently emit light.Excitation energy is determined by emission spectroscopy.

The host material may be a mixture of a plurality species of hostmaterials. Part of the host material 204 may be substituted withfluoroalkyl, perfluoroalkyl, alkyl (of 10 carbon atoms or more),perfluoropolyether or siloxy (—Si—O—Si—) group. Such a material, whenincorporated in the host material 204, facilitates localization of partof the host material in the light-emitting layer 203 surface, with theresult that it is present in the surface together with thered-light-emitting dopant to prevent agglomeration of the dopant andthereby to improve light-emitting efficiency.

This example used an iridium complex represented by Formula 3′ for thegreen-light-emitting dopant.

An iridium complex having 2 species of different ligands, represented byFormula 4′ may be used instead of a coumarin compound or the aboveiridium complex having almost similar ligands.

The green-light-emitting dopant does not need a special functionalgroup. It is however preferably low in symmetry viewed from improvingsolubility. It may have a substituent low in compatibility with theunderlying layer, such as the hole transport layer, hole injection layeror lower electrode.

This example used an iridium complex represented by Formula 5′ having asubstituent for the blue-light-emitting dopant for transferring thedopant toward the underlying layer.

The functional group varies depending on the underlying layer to whichthe dopant is transferred during the film-making step. When theunderlying layer is the hole transport layer, the group should have astructure similar to the hole transport layer, e.g., arylamino orcarbazole group, or hydrazone site. When the underlying layer is of ITOor a metal, examples of the functional group include hydroxyl (—OH),thiol (—SH), carboxyl (—COOH), sulfo (—SO₃H) or bipyridyl group, or I,Br, Cl, F, SCN, CN, NH₂ or NO₂. The green-light-emitting dopant 207 maycontain one or more of these functional groups. Hydroxyl is preferable,when the light-emitting layer is incorporated with an oxide of highspecific gravity. A long-chain alkyl group is preferable, when theunderlying layer has a long-chain alkyl group. The group may beintroduced to the major skeleton directly, as illustrated by Formula 5′,or indirectly via an alkyl chain in consideration of the molecular size.A perylene derivative is also a useful material for theblue-light-emitting dopant 207.

Next, the other components are described. As discussed earlier, theorganic layer 213 may not necessarily need the hole injection layer 201,hole transport layer 202, electron transport layer 208 or electroninjection layer 209.

This example used poly(3,4-ethylenedioxythiophene (PEDOT) andpolystyrenesulfonate (PSS) for the hole injection layer 201.Polyaniline-base, polypyrrole-base and triphenylamine-base polymers arealso useful. A material incorporated with fine metal particles is alsouseful. They may be used in combination of low-molecular-weightmaterial. A phthalocyanine-base compound is also applicable.

This example used an arylamine-base polymer for the hole transport layer202. Other materials useful for the hole transport layer 202 include,but not limited to, polyfluorene-base, polyparaphenylene-base,polyarylene-base and polycarbazole-base polymers, andstarburst-amine-base compound, stilbene derivative, hydrazone derivativeand thiophene derivative. They may be used in combination.

The electron transport layer 208 was responsible for donating electronsto the light-emitting layer 203. This example usedbis(2-methyl-8-quinolinolate)-4-(phenylphenolate) aluminium (Balq) andtris(8-quinolinolate) aluminum (Alq3), which formed a laminatedstructure, for the electron transport layer 208. This layer may be of asingle-layer structure of Balq, Alq3, oxadiazole derivative, fllerenederivative, quinoline derivative or silole derivative.

The electron injection layer 209 worked to improve efficiency ofinjection of the electrons from the cathode to the electron transportlayer 208. This example used lithium fluoride for the layer 209. Otheruseful materials include, but not limited to, magnesium fluoride,calcium fluoride, strontium fluoride, barium fluoride, magnesium oxideand aluminum oxide. A mixture of electron-transferring material andalkali metal or alkali metal oxide, or mixture of electron-transferringmaterial and electron-donating material may be used. They may be used incombination.

This example used ITO for the lower electrode 211. The material for theanode as the lower electrode 211 is not limited so long as it istransparent and having a high work function. The examples of thematerial include an electroconductive oxide, e.g., ITO or IZO, or metalof high work function, e.g., thin Ag. The electrode can be patternednormally on a substrate, e.g., glass, by photolithography.

This example used Al for the upper electrode 212. The cathode as theupper electrode 212 injected the electrons into the light-emitting layer203 to reflect light emitted from the light-emitting layer. Examples ofthe suitable material for the upper electrode 212 specifically includeAl, Mg/Ag alloy and Ag.

The coating solution was composed of the host material 204,red-light-emitting dopant 205, green-light-emitting dopant 206 andblue-light-emitting dopant 207, dissolved in an adequate solvent. Thecoating solution may not necessarily need the green-light-emittingdopant 206. The solution contained the host material 204,red-light-emitting dopant 205, green-light-emitting dopant 206 andblue-light-emitting dopant 207 in a ratio of 100/1/5/1 by mass. In termsof the concentration based on the solid component, thered-light-emitting dopant 205, green-light-emitting dopant 206 andblue-light-emitting dopant 207 were present at 0.46, 2.9 and 0.5% bymol, respectively. This example used tetrahydrofuran (THF) as thesolvent. The solvents useful for the present invention are not limitedso long as they can dissolve these components. The examples includearomatic hydrocarbons (e.g., toluene), ethers (e.g., tetrahydrofuran),alcohols, fluorine-base ones. These solvents may be used in combinationfor adjusting solubility and drying speed of each component. Forexample, a mixture of solvents of different boiling point may be used,wherein the higher-boiling one is used as a poor solvent with thered-light-emitting dopant 205 to accelerate transfer of the dopant tothe film surface. Solubility of the solvent is determined by liquidchromatography.

This example used spin coating for forming the light-emitting layer. Theother useful methods include casting, dip coating, spray coating, screenprinting and ink-jet printing.

The device structure is described taking a bottom emission typestructure as an example. However, the present invention is alsoapplicable to a top emission type structure with the upper electrodeworking as a transparent electrode so long as the upper and lowerelectrodes work as the respective cathode and anode.

A voltage was applied to the light-emitting device prepared in thisexample, positive at the lower electrode and negative at the upperelectrode, to confirm that the device emitted white light comprisingblue, green and red lights. FIG. 4 illustrates the energy diagram of thedevice.

The device has the energy diagram illustrated in FIG. 4, when each ofthe red-light-emitting dopant 205 and blue-light-emitting dopant 207 hasspontaneous phase-separation. FIG. 4 shows that the lowest unoccupiedmolecular orbital (LUMO) energy of the blue-light-emitting dopant 207 isintermediate between those of the light-emitting layer 203 and secondelectrode 211, and lower than that of the layer adjacent to thelight-emitting layer 203. The lowest unoccupied molecular orbital (LUMO)energy is determined by finding the HOMO (highest occupied molecularorbital)-LUMO differential energy from the absorption spectra ordirectly by inverse photoemission spectroscopy. In this case, theelectron is injected from the electron transport layer 208 into thelight-emitting layer 203, and trapped by the blue-light-emitting dopant207. On the other hand, the hole is injected from the hole transportlayer 202 into the blue-light-emitting dopant 207, where the electronand hole are recombined with each other to emit light. Part of the holeis transferred toward the green-light-emitting dopant 206 andred-light-emitting dopant 205, where it is recombined with the electroninjected from the electron transport layer 208 to emit light. Thus, thedevice efficiently emits light, because of the presence of energybarrier which blocks flow of the electron between the hole transportlayer 202 and blue-light-emitting dopant 207.

Comparative Example 1

Comparative Example 1 prepared a light-emitting device in the samemanner as in Example 3, except that a compound represented by Formula 6′was used for the red-light-emitting dopant 205 and compound representedby Formula 7′ for the blue-light-emitting dopant 207. The device emittedlight of high intensity only from the red-light-emitting dopant 205,with the blue-light-emitting dopant 207 and green-light-emitting dopant206 emitting light of low intensity, conceivably because thegreen-light-emitting dopant 206 and red-light-emitting dopant 205 wereclose enough to the blue-light-emitting dopant 207 to cause transfer ofenergy from the blue-light-emitting dopant 207 and green-light-emittingdopant 206 to the red-light-emitting dopant 205.

Example 4

Example 4 prepared a light-emitting device in the same manner as inExample 3, except that the compound represented by Formula 7′ was usedfor the blue-light-emitting dopant 207 for the light-emitting layer andthe coating solution contained the host material 204, red-light-emittingdopant 205, green-light-emitting dopant 206 and blue-light-emittingdopant 207 in a ratio of 100/1/0.5/1 by mass. The red-light-emittingdopant 205, green-light-emitting dopant 206 and blue-light-emittingdopant 207 were present at 0.45, 0.28 and 5.5% by mol, respectively. Thedevice emitted white light comprising blue, green and red lights.

Comparative Example 2

Comparative Example 2 prepared a light-emitting device in the samemanner as in Example 4, except that the compound represented by Formula6′ was used for the red-light-emitting dopant 205. The device emittedlight of high intensity only from the red-light-emitting dopant 205,with the blue-light-emitting dopant 207 and green-light-emitting dopant206 emitting light of low intensity.

Example 5

Example 5 prepared a light-emitting device in the same manner as inExample 3, except that a compound represented by Formula 8′ was used inaddition to mCP for the host material 204 for the light-emitting layer.

The device emitted light more efficiently than that prepared in Example3.

Example 6

Example 6 prepared a light-emitting device in the same manner as inExample 4, except that a compound represented by Formula 8′ was used inaddition to mCP for the host material 204 for the light-emitting layer.

The device emitted light more efficiently than that prepared in Example4.

Example 7

Example 7 prepared a light-emitting device in the same manner as inExample 3, except that a compound represented by Formula 9′ was used forthe red-light-emitting dopant 205 for the light-emitting layer. Thedevice emitted white light comprising blue, green and red lights.

Example 8

Example 8 prepared a light-emitting device in the same manner as inExample 3, except that a compound represented by Formula 10′ was usedfor the blue-light-emitting dopant 207 for the light-emitting layer, andthe light-emitting layer contained fine titania particles having adiameter of about 2 nm.

The device emitted white light comprising blue, green and red lights.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

DESCRIPTION OF REFERENCE NUMERALS

-   101 Hole injection layer-   102 Hole transport layer-   103 Light-emitting layer-   104 Host material-   105 Red-light-emitting dopant-   106 Green-light-emitting dopant-   107 Blue-light-emitting dopant-   108 Electron transport layer-   109 Electron injection layer-   110 Substrate-   111 Lower electrode-   112 Upper electrode-   113 Organic layer-   201 Hole injection layer-   202 Hole transport layer-   203 Light-emitting layer-   204 Host material-   205 Red-light-emitting dopant-   206 Green-light-emitting dopant-   207 Blue-light-emitting dopant-   208 Electron transport layer-   209 Electron injection layer-   210 Substrate-   211 Lower electrode-   212 Upper electrode

1. An organic light-emitting device comprising: a first electrode; a second electrode; and a light-emitting layer positioned between the first electrode and the second electrode, wherein the light-emitting layer contains a host material, a red-light-emitting dopant and a blue-light-emitting dopant, the red-light-emitting dopant has a first functional group for transferring the dopant toward the first electrode, and the host material has a higher excitation energy than the blue-light-emitting dopant.
 2. The organic light-emitting device according to claim 1, wherein the first functional group is at least one species selected from the group consisting of fluoroalkyl, perfluoroalkyl, alkyl (of 10 carbon atoms or more), perfluoropolyether and siloxy groups.
 3. The organic light-emitting device according to claim 1, wherein the red-light-emitting dopant is of the iridium complex represented by Formula 1

wherein X1 is an N-containing aromatic hetero ring, X2 is an aromatic hydrocarbon ring or aromatic hetero ring, and Y1 and Y2 are each fluoroalkyl, perfluoroalkyl, alkyl, perfluoropolyether or siloxy groups.
 4. The organic light-emitting device according to claim 1, wherein the light-emitting layer contains a green-light-emitting dopant which has a second functional group for transferring the dopant to the second electrode.
 5. The organic light-emitting device according to claim 4, wherein the second functional group is at least one species selected from the group consisting of hydroxyl (—OH), thiol (—SH), carboxyl (—COOH), sulfo (—SO₃H), I, Br, Cl, F, SCN, CN, NH₂, NO₂ and bipyridyl groups.
 6. The organic light-emitting device according to claim 4, wherein a hole injection layer is disposed between the second electrode and the light-emitting layer, and the second functional group is at least one species selected from the group consisting of phenylamino, oxazole and carbazole group, and hydrazone site.
 7. The organic light-emitting device according to claim 4, wherein the absolute value of the lowest unoccupied molecular orbital energy of the blue-light-emitting dopant is higher than that of the green-light-emitting dopant.
 8. The organic light-emitting device according to claim 1, wherein the light-emitting layer is prepared by a coating method.
 9. A coating solution used for forming the light-emitting layer for the organic light-emitting device according to claim 1, containing a solvent, host material, red-light-emitting dopant and blue-light-emitting dopant.
 10. A coating solution used for forming the light-emitting layer for the organic light-emitting device according to claim 4, containing a solvent, host material, red-light-emitting dopant, blue-light-emitting dopant and green-light-emitting dopant.
 11. The coating solution according to claim 9, wherein the solvent contains first and second solvents, the first solvent boiling at a higher temperature than the second one, and working as a poor solvent with the red-light-emitting dopant.
 12. A luminescent material used for forming the light-emitting layer for the organic light-emitting device according to claim 1, containing a host material, red-light-emitting dopant and blue-light-emitting dopant.
 13. A luminescent material used for forming the light-emitting layer for the organic light-emitting device according to claim 4, containing a host material, red-light-emitting dopant, blue-light-emitting dopant and green-light-emitting dopant.
 14. A light source device using the organic light-emitting device according to claim
 1. 15. An organic light-emitting device comprising: a first electrode; a second electrode; and a light-emitting layer positioned between the first electrode and the second electrode, wherein the light-emitting layer contains a host material, a red-light-emitting dopant and a green-light-emitting dopant, the red-light-emitting dopant is present in the light-emitting layer at graded concentrations, the red-light-emitting dopant has a first substituent to be localized in the vicinity of the first electrode, and the host material has a higher excitation energy than the blue-light-emitting dopant.
 16. The organic light-emitting device according to claim 15, wherein the first substituent contains at least one species of functional group selected from the group consisting of fluoroalkyl, perfluoroalkyl, alkyl (of 10 carbon atoms or more), perfluoropolyether and siloxy groups.
 17. The organic light-emitting device according to claim 15, wherein the red-light-emitting dopant is of the iridium complex represented by Formula 2′

wherein X1 is an N-containing aromatic hetero ring, X2 is an aromatic hydrocarbon ring or aromatic hetero ring, and Y1 and Y2 are each fluoroalkyl, perfluoroalkyl, alkyl (of 10 carbon atoms or more), perfluoropolyether or siloxy (Si—O—Si—) group.
 18. The organic light-emitting device according to claim 15, wherein the light-emitting layer contains a green-light-emitting dopant which contains a second substituent to be transferred to the second electrode.
 19. The organic light-emitting device according to claim 15, wherein the second substituent contains at least one species of functional group selected from the group consisting of arylamino and carbazole groups, and hydrazone site.
 20. The organic light-emitting device according to claim 15, wherein a hole transport layer is disposed between the second electrode and light-emitting layer, and the second substituent contains is at least one species of functional group selected from the group consisting of hydroxyl, thiol, carboxyl, sulfo, I, Br, Cl, F, SCN, CN, NH₂, NO₂ and bipyridyl groups.
 21. The organic light-emitting device according to claim 15, wherein the host material contains the first substituent.
 22. A coating solution used for forming the light-emitting layer for the organic light-emitting device according to claim 15, containing a solvent, host material, red-light-emitting dopant and green-light-emitting dopant.
 23. A coating solution used for forming the light-emitting layer for the organic light-emitting device according to claim 18, containing a solvent, host material, red-light-emitting dopant, green-light-emitting dopant and blue-light-emitting dopant.
 24. The organic light-emitting device according to claim 15, containing fine particles of metal oxide.
 25. The organic light-emitting device according to claim 18, wherein the blue-light-emitting dopant has the lowest unoccupied molecular orbital energy intermediate between those of the light-emitting layer and second electrode, and lower than that of the layer adjacent to the light-emitting layer.
 26. A luminescent material used for forming the light-emitting layer for the organic light-emitting device according to claim 15, containing a host material, red-light-emitting dopant and green-light-emitting dopant.
 27. A luminescent material used for forming the light-emitting layer for the organic light-emitting device according to claim 18, containing a host material, red-light-emitting dopant, blue-light-emitting dopant and green-light-emitting dopant.
 28. A method for making an organic light-emitting device comprising a first electrode, a second electrode and a light-emitting layer positioned between the first electrode and the second electrode, the light-emitting layer containing a host material, a red-light-emitting dopant and a blue-light-emitting dopant, the red-light-emitting dopant has a first functional group for transferring the dopant toward the first electrode, and the host material has a higher excitation energy than the blue-light-emitting dopant, the method comprising: providing the first electrode; coating a coating solution comprising a solvent, host material, red-light-emitting dopant and blue-light-emitting dopant over the first electrode, and forming the light-emitting layer containing the host material, the a red-light-emitting dopant and the blue-light-emitting dopant over the first electrode from the coating solution; and providing the second electrode over the light-emitting layer.
 29. The method according to claim 28, wherein the coating of the coating solution on the first electrode comprises a coating method selected from the group consisting of spin coating, casting, dip coating, spray coating, screen printing and ink jet printing. 